Common examples of powder coatings include epoxy-series, acryl-series and polyester-series. It is well known that polyester-series powder coatings have well-balanced coating performance.
Powder coatings have been widely used in various fields including appliance, automobile and construction materials. In some cases, a glossy coated surface (specular gloss at 60.degree.: approximately 90 to 100%) is required for aesthetic purposes, while a mat (specular gloss at 60.degree.: approximately less than 30%) or semimat (specular gloss at 60.degree.: approximately 30 to 70%) may be required for other purposes.
Examples of conventional methods for preparing a powder coating capable of giving such a mat or semimat coating film include a method comprising adding a large amount of a pigment of coarse particles, and another method comprising dry-blending a polyester-series powder coating with an acryl-series powder coating.
However, the addition of a large amount of a pigment of coarse particles causes a deterioration in smoothness or a decrease in mechanical strength of the coating film. On the other hand, when using the method of dry-blending the two powder coatings, a mat or semimat powder coating cannot be obtained by a single step, which increases production cost. Furthermore, this method suffers from other problems including heterogeneity of the resulting blend and a change in the blending ratio during recovery and application.
In order to solve the above-mentioned problems, JP-A-64-1770 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a resin composition for a powder coating comprising a polyester having a hydroxyl value of at least 1,200 g eq/10.sup.6 g, another polyester having a hydroxyl value of from 200 to 1,000 g eq/10.sup.6 g and a blocked isocyanate hardener wherein the difference in gel time between the two polyesters is at least 3 minutes.
In the case of a powder coating comprising this resin composition for a powder coating, however, a large amount of a di- or tri-functional blocked isocyanate hardener should be blended in order to control the functional value to a level almost equivalent with the hydroxyl value, since a polyester having a hydroxyl value of at least 1,200 g eq/10.sup.6 g is contained therein. As a result, there occur some troubles, for example, contamination of a stove or a material to be coated due to vaporization of a large amount of a blocking agent during the reaction between the polyester and the blocked isocyanate hardener and a weight loss of 10% or above.